For me 3-4x more is definitely not worth. Sin to Bangkok is only 2hrs. It's not worth for such a short trip.
When I compare my flights I often find budget flights to not have much cost savings after adding add-on. If it's only $100-200+ more for full service I will opt for full service. The only time I took long haul budget is when I booked scoot to Hokkaido at half the cost of full service ANA/JAL.